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Why Exercise Timing Matters — But Not for the Reason You Think

Nov 16, 2012

A recent study published online in October 2012 in the British Journal of Nutrition (and reported on in Diabetes In Control) attempted to address the issue of how timing of exercise relative to meal ingestion influences substrate balance and metabolic responses1.

In that study, ten sedentary, overweight men with a mean age of 28 years undertook an hour of moderate walking either before or after eating breakfast with their fat balance and postprandial metabolism monitored afterwards. Not surprisingly, exercising at either time (compared to just resting) significantly lowered insulin responses. Moreover, since more total fat was oxidized during the 8.5-hour period subjects were followed (which included the exercise period) and blood levels of triglycerides (blood fats) remained lower following pre-breakfast exercise in particular, the authors suggested that there may be an advantage for body fat regulation and lipid metabolism gained by exercising before compared with after breakfast.


The conclusions jumped to, based on studies like this, personally drive me crazy, especially when they are assumed to apply equally to people with diabetes (even though this study was done on overweight young men without diabetes). Despite the conclusions given in the article suggesting advantages in fat metabolism and weight loss, in actuality all the differences in calorie and substrate utilization over the ensuing 8.5 hours post-exercise were entirely accounted for by energy use during the exercise sessions themselves, not differences following the activity. In other words, walking before breakfast caused them to use (slightly) more fat during the activity — not surprisingly — than walking after their morning meal. When interviewed about these findings, Dr. Gill apparently said that while exercise in itself is good, any done before breakfast may be extra beneficial because it forces the body to rely on its stores of fat for energy.

Urgh!! That study was obviously not undertaken by or reported on by anyone with a background in exercise physiology. Conclusions like these perpetuate the myth that you have to “burn” fat to lose body fat, which is completely and utterly not true. While the loss of body fat undeniably requires a negative calorie balance, the operative word is calorie, not fat. Larger body fat losses result from a greater total calorie use, regardless of which fuels are used during the activity. Using a little bit of extra fat during a pre-breakfast walk is not going to make people lose more body fat — it’s the calorie deficit that contributes to weight loss! What’s more, exercising when carbohydrate stores are limited after an overnight fast will likely result in a smaller calorie deficit since you are less likely to be able to exercise hard and/or long without eating first.

For moderate or harder exercise, carbohydrate is the body’s preferred fuel, not fat, because the former is more fuel efficient. Having to rely more heavily on fat causes you to slow your pace and/or limit your total activity. Fat, however, is the body’s primary fuel during recovery from any exercise, which spares blood glucose for glycogen restoration and amino acids (from protein) for muscle repair and more. Thus, most of the calories supplying your body’s basal energy needs during the day come from fat, which is when body fat losses are greatest.

For those with diabetes, engaging in pre-breakfast exercise can actually be detrimental to blood glucose management. Anyone who exercises first thing in the morning before eating prolongs the fasting period and promotes an even greater release of glucose-raising hormones like cortisol, which in particular heightens insulin resistance pre-breakfast. In fact, a usual response for anyone with diabetes who exercises before eating breakfast is an increase in blood glucose levels, not the usual decline experiences during moderate physical activity. I can’t tell you how many times I have advised people with diabetes to eat something first before engaging in early morning exercise to prevent elevations in their blood glucose levels!

When it comes to preventing postprandial spikes in blood glucose, exercising after dinner is much more effective than doing it before2. Others have shown that for people with type 2 diabetes, a prior meal helps enhance the glucose-lowering effect of physical activity3. So, when it comes to both body weight and blood glucose management, exercise done at any other time of day is likely better advice for the person with diabetes than exercising in a fasted state first thing in the morning. Remember, you do not have to “burn” fat during exercise to effectively lose body fat!

  1. Farah NMF, Gill JMR: Effects of exercise before or after meal ingestion on fat balance and postprandial metabolism in overweight men. British J Nutrition 2012 (Oct):1-11; available on CJO2012 doi:10.1017/S0007114512004448
  2. Colberg SR, Zarrabi L, Bennington L, Nakave A, Thomas Somma C, Swain DP, Sechrist SR: Postprandial walking is better for lowering the glycemic effect of dinner than pre-dinner exercise in type 2 diabetic individuals. J Am Med Dir Assoc 2009;10:394-397
  3. Poirier P, Mawhinney S, Grondin L, Tremblay A, Broderick T, Cleroux J, Catellier C, Tancrede G, Nadeau A: Prior meal enhances the plasma glucose lowering effect of exercise in type 2 diabetes. Med Sci Sports Exerc 2001;33:1259-1264

To manage or prevent type 2 diabetes in youth, consult Diabetes-Free Kids: A Take-Charge Plan for Preventing and Treating Type 2 Diabetes in Youth. More information about this book and where to order it online can be found at www.shericolberg.com, along with other tips and books for active and sedentary adults with diabetes or prediabetes.